This invention relates to a screw actuator of the type in which a first component is rotatable, a second component being held non-rotatably, the first and second components cooperating with one another such that rotation of the first component causes the second component to move axially relative to the first component.
In some applications, for example where the actuator is used to move a cowl of a thrust reverser system, the actuator may, under some circumstances, have applied thereto an assisting or aiding load or force. It is desirable to avoid transmitting such loads or forces to the motor used to drive the actuator. It is an object of the invention to provide an actuator in which the disadvantage described hereinbefore is of reduced effect.
According to the present invention there is provided a screw actuator comprising first and second relatively rotatable members, a screw thread formation in said first member including a helical groove, spherical elements carried by the second member and in rolling engagement in said helical groove to form a ball screw coupling between the first and second members, said groove being of sufficient width to allow the spherical elements to move laterally within the groove, and a brake member carried by the second member and defining a screw thread formation which is received in said groove of the screw thread formation of the first member, the brake member being angularly moveable relative to the second member by rotation of the first member relative to the second member, between a first angular position in which the screw thread formation of the brake member is cooperable with that of the first member, and a second angular position in which the screw thread formation of the brake member is not cooperable with that of the first member.
Preferably said brake member will occupy said first angular position in response to rotation of the first member relative to said second member in one direction and during such rotation the thread formation of the brake member will brakingly engage the thread formation of the first member when the actuator is subject to an axial load in one direction but not when the actuator is subject to an axial load in the opposite direction, and, said brake member will occupy said second angular position in response to rotation of the first member relative to said second member in the opposite direction and during such rotation the thread formation of the brake member will not brakingly engage the thread formation of the first member irrespective of the direction in which axial load is imposed on the actuator.
In such an arrangement, during rotation of the first member in the said one direction, whilst a compressive load is applied to the actuator as would occur when the actuator is driving a thrust reverser cowl out of its stowed position, the reaction force to the compressive load is transmitted through the spherical elements and the ball screw coupling provides an efficient drive between the first and second members. In the event that a tensile load is applied to the actuator, for example as a result of the cowl applying an aiding or assisting load to the actuator, the reaction force to the tensile load is applied through the brake member, and the effect of friction between the brake member and the first member will cause the actuator to operate less efficiently. As a result, the aiding or assisting load is not transmitted through the actuator to an extent sufficient to drive and cause damage to the motor.
When the first member is rotated relative to the second member in the reverse direction, the brake member moves to an angular position in which the thread formation thereof is either out of contact with the thread formation of the first member or is free to slide, with little resistance, relative to the thread formation of the first member, and the drive will be relatively efficient regardless as to the direction of any load applied to the actuator as the reaction force to any such load will be applied through the spherical elements.